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+# Copyright (C) 1996,1998 A. Scottedward Hodel 
+#
+# This file is part of Octave. 
+#
+# Octave is free software; you can redistribute it and/or modify it 
+# under the terms of the GNU General Public License as published by the 
+# Free Software Foundation; either version 2, or (at your option) any 
+# later version. 
+# 
+# Octave is distributed in the hope that it will be useful, but WITHOUT 
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License 
+# for more details.
+# 
+# You should have received a copy of the GNU General Public License 
+# along with Octave; see the file COPYING.  If not, write to the Free 
+# Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. 
+ 
+function sys = sysgroup(Asys,Bsys)
+# function sys = sysgroup(Asys,Bsys)
+# Combines two system data structures into a single system
+#
+# input: Asys, Bsys: system data structures
+# output: sys: Asys and Bsys are combined into a single system:
+#
+#              __________________
+#              |    ________    |
+#     u1 ----->|--> | Asys |--->|----> y1
+#              |    --------    |
+#              |    ________    |
+#     u2 ----->|--> | Bsys |--->|----> y2
+#              |    --------    |
+#              ------------------
+#                   Ksys
+# 
+# The function also rearranges the A,B,C matrices so that the 
+# continuous states come first and the discrete states come last.
+# If there are duplicate names, the second name has a unique suffix appended
+# on to the end of the name.
+
+# A. S. Hodel August 1995
+# modified by John Ingram July 1996
+# $Revision: 1.3 $
+
+  save_val = implicit_str_to_num_ok;	# save for later
+  implicit_str_to_num_ok = 1;
+
+  save_emp = empty_list_elements_ok;
+  empty_list_elements_ok = 1;
+
+  if(nargin ~= 2)
+    usage("sys = sysgroup(Asys,Bsys)");
+  elseif( !is_struct(Asys) | !is_struct(Bsys) )
+    error("sysgroup: input arguments must both be structured systems");
+  endif
+
+  # extract information from Asys, Bsys to consruct sys
+  Asys = sysupdate(Asys,"ss");
+  Bsys = sysupdate(Bsys,"ss");
+  [n1,nz1,m1,p1] = sysdimensions(Asys);
+  [n2,nz2,m2,p2] = sysdimensions(Bsys);
+  [Aa,Ab,Ac,Ad,Atsam,An,Anz,Ast,Ain,Aout,Ayd] = sys2ss(Asys);
+  [Ba,Bb,Bc,Bd,Btsam,Bn,Bnz,Bst,Bin,Bout,Byd] = sys2ss(Bsys);
+  nA = An + Anz;
+  nB = Bn + Bnz;
+
+  if(p1*m1*p2*m2 == 0)
+    error("sysgroup: argument lacks inputs and/or outputs");
+
+  elseif((Atsam + Btsam > 0) & (Atsam * Btsam == 0) )
+    warning("sysgroup: creating combination of continuous and discrete systems")
+
+  elseif(Atsam != Btsam)
+    error(["sysgroup: Asys.tsam=", ...
+       num2str(Atsam),", Bsys.tsam=",num2str(Btsam)]);
+
+  endif
+
+  A = [Aa,zeros(nA,nB); zeros(nB,nA),Ba];
+  B = [Ab,zeros(nA,m2); zeros(nB,m1),Bb];
+  C = [Ac,zeros(p1,nB); zeros(p2,nA),Bc];
+  D = [Ad,zeros(p1,m2); zeros(p2,m1),Bd];
+  tsam = max(Atsam,Btsam);
+
+  # construct combined signal names; stnames must check for pure gain blocks
+  if(isempty(Ast))
+    stname = Bst;
+  elseif(isempty(Bst))
+    stname = Ast;
+  else
+    stname  = str2mat(Ast, Bst);
+  endif
+  inname  = str2mat(Ain, Bin);
+  outname = str2mat(Aout,Bout);
+
+  # Sort states into continous first, then discrete
+  dstates = ones(1,(nA+nB));
+  if(An)
+    dstates(1:(An)) = zeros(1,An);
+  endif
+  if(Bn)
+    dstates((nA+1):(nA+Bn)) = zeros(1,Bn);
+  endif
+  [tmp,pv] = sort(dstates);
+  A = A(pv,pv);
+  B = B(pv,:);
+  C = C(:,pv);
+  stname = stname(pv,:);
+
+  # check for duplicate signal names
+  inname = sysgroupn(inname,"input");
+  stname = sysgroupn(stname,"state");
+  outname = sysgroupn(outname,"output");
+
+  # mark discrete outputs
+  outlist = find([Ayd, Byd]);
+
+  # build new system
+  sys = ss2sys(A,B,C,D,tsam,An+Bn,Anz+Bnz,stname,inname,outname);
+
+  implicit_str_to_num_ok = save_val;	# restore value  
+  empty_list_elements_ok = save_emp;
+
+endfunction